Phenotypic plasticity and evolution of thermal tolerance in bacteria from temperate and hot spring environments

Is Bacillus cytotoxicus from edible insects a threat?

Bacillus cytotoxicus is considered a potential emerging foodborne pathogen that has been under investigation in recent years. Most studies have focused on strains from vegetables, particularly potato products, but there is limited information on strains from other food sources. This study addresses the current research gap by investigating the genomic and phenotypic features of B. cytotoxicus isolated from edible insects. The whole genomes and key phenotypic traits of 20 strains isolated from edible insects were investigated. The comparative genome analysis also included 44 available genomes from other sources to identify possible genetic links and the mosaicism of virulence profiles (VP) and antimicrobial resistance genes (AMR). B. cytotoxicus isolated from edible insects showed marked thermotolerance, when vegetative forms could grow at 50-60 °C and survive at 65 °C and exhibited marked proteolytic activities, even at higher temperatures. The heterogeneous phenotypes observed suggest potential issues with defining suitable protocols for isolation and identification in this food matrix. Despite the limited genomic diversity observed, it was possible to identify links between isolates, demonstrating the co-isolation of different genomes/phenotypes from various insect samples and suggesting trade links between insect companies and the persistence of certain strains. A genomic comparison suggested segregating strains from edible insects with similar VP and AMR profiles. These findings indicate a degree of adaptation to different food niches, with strains from insects or insect-based products differing partially from those isolated from vegetable sources, showing possible associations with their respective food environments. The survival advantage conferred by thermotolerance underscores the need to assess the presence of these spore-forming bacteria carefully and to calibrate treatments and processes, to address the emerging risk posed by this pathogen and its implications for food safety.

Thermal Plasticity and Evolutionary Constraints in Bacillus: Implications for Climate Change Adaptation

The ongoing rise in global temperatures poses significant challenges to ecosystems, particularly impacting bacterial communities that are central to biogeochemical cycles. The resilience of wild mesophilic bacteria to temperature increases of 2-4 °C remains poorly understood. In this study, we conducted experimental evolution on six wild Bacillus strains from two lineages (Bacillus cereus and Bacillus subtilis) to examine their thermal adaptation strategies. We exposed the bacteria to gradually increasing temperatures to assess their thermal plasticity, focusing on the genetic mechanisms underlying adaptation. While B. subtilis lineages improved growth at highly critical temperatures, only one increased its thermal niche to 4 °C above their natural range. This finding is concerning given climate change projections. B. cereus strains exhibited higher mutation rates but were not able to grow at increasing temperatures, while B. subtilis required fewer genetic changes to increase heat tolerance, indicating distinct adaptive strategies. We observed convergent evolution in five evolved lines, with mutations in genes involved in c-di-AMP synthesis, which is crucial for potassium transport, implicating this chemical messenger for the first time in heat tolerance. These insights highlight the vulnerability of bacteria to climate change and underscore the importance of genetic background in shaping thermal adaptation.

Survival strategies of Bacillus spp. in saline soils: Key factors to promote plant growth and health

Soil salinity is one of the most important abiotic factors that affects agricultural production worldwide. Because of saline stress, plants face physiological changes that have negative impacts on the various stages of their development, so the employment of plant growth-promoting bacteria (PGPB) is one effective means to reduce such toxic effects. Bacteria of the Bacillus genus are excellent PGPB and have been extensively studied, but what traits makes them so extraordinary to adapt and survive under harsh situations? In this work we review the Bacillus' innate abilities to survive in saline stressful soils, such as the production osmoprotectant compounds, antioxidant enzymes, exopolysaccharides, and the modification of their membrane lipids. Other survival abilities are also discussed, such as sporulation or a reduced growth state under the scope of a functional interaction in the rhizosphere. Thus, the most recent evidence shows that these saline adaptive activities are important in plant-associated bacteria to potentially protect, direct and indirect plant growth-stimulating activities. Additionally, recent advances on the mechanisms used by Bacillus spp. to improve the growth of plants under saline stress are addressed, including genomic and transcriptomic explorations. Finally, characterization and selection of Bacillus strains with efficient survival strategies are key factors in ameliorating saline problems in agricultural production.

Agroecological Management of the Grey Mould Fungus Botrytis cinerea by Plant Growth-Promoting Bacteria

Botrytis cinerea is the causal agent of grey mould and one of the most important plant pathogens in the world because of the damage it causes to fruits and vegetables. Although the application of botrycides is one of the most common plant protection strategies used in the world, the application of plant-beneficial bacteria might replace botrycides facilitating agroecological production practices. Based on this, we reviewed the different stages of B. cinerea infection in plants and the biocontrol mechanisms exerted by plant-beneficial bacteria, including the well-known plant growth-promoting bacteria (PGPB). Some PGPB mechanisms to control grey mould disease include antibiosis, space occupation, nutrient uptake, ethylene modulation, and the induction of plant defence mechanisms. In addition, recent studies on the action of anti-Botrytis compounds produced by PGPB and how they damage the conidial and mycelial structures of the pathogen are reviewed. Likewise, the advantages of individual inoculations of PGPB versus those that require the joint action of antagonist agents (microbial consortia) are discussed. Finally, it should be emphasised that PGPB are an excellent option to prevent grey mould in different crops and their use should be expanded for environmentally friendly agricultural practices.